一 介紹

本節主題

多表連接查詢

復合條件連接查詢

子查詢

準備表

company.employee
company.department

#建表 create table department( id int, name varchar(20) );create table employee( id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int );#插入數據 insert into department values (200,'技術'), (201,'人力資源'), (202,'銷售'), (203,'運營');insert into employee(name,sex,age,dep_id) values ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ('liwenzhou','male',18,200), ('jingliyang','female',18,204) ;#查看表結構和數據 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+mysql> desc employee; +--------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(11) | YES | | NULL | | | dep_id | int(11) | YES | | NULL | | +--------+-----------------------+------+-----+---------+----------------+mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技術 | | 201 | 人力資源 | | 202 | 銷售 | | 203 | 運營 | +------+--------------+mysql> select * from employee; +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+

二 多表連接查詢

#重點：外鏈接語法 SELECT 字段列表FROM 表1 INNER|LEFT|RIGHT JOIN 表2ON 表1.字段 = 表2.字段;

1 交叉連接：不適用任何匹配條件。生成笛卡爾積

mysql> select * from employee,department; # +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技術 | | 1 | egon | male | 18 | 200 | 201 | 人力資源 | | 1 | egon | male | 18 | 200 | 202 | 銷售 | | 1 | egon | male | 18 | 200 | 203 | 運營 | | 2 | alex | female | 48 | 201 | 200 | 技術 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | | 2 | alex | female | 48 | 201 | 202 | 銷售 | | 2 | alex | female | 48 | 201 | 203 | 運營 | | 3 | wupeiqi | male | 38 | 201 | 200 | 技術 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 | | 3 | wupeiqi | male | 38 | 201 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 201 | 203 | 運營 | | 4 | yuanhao | female | 28 | 202 | 200 | 技術 | | 4 | yuanhao | female | 28 | 202 | 201 | 人力資源 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 4 | yuanhao | female | 28 | 202 | 203 | 運營 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | | 5 | liwenzhou | male | 18 | 200 | 201 | 人力資源 | | 5 | liwenzhou | male | 18 | 200 | 202 | 銷售 | | 5 | liwenzhou | male | 18 | 200 | 203 | 運營 | | 6 | jingliyang | female | 18 | 204 | 200 | 技術 | | 6 | jingliyang | female | 18 | 204 | 201 | 人力資源 | | 6 | jingliyang | female | 18 | 204 | 202 | 銷售 | | 6 | jingliyang | female | 18 | 204 | 203 | 運營 | +----+------------+--------+------+--------+------+--------------+

2 內連接：只連接匹配的行

#找兩張表共有的部分，相當于利用條件從笛卡爾積結果中篩選出了正確的結果 #department沒有204這個部門，因而employee表中關于204這條員工信息沒有匹配出來 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; +----+-----------+------+--------+--------------+ | id | name | age | sex | name | +----+-----------+------+--------+--------------+ | 1 | egon | 18 | male | 技術 | | 2 | alex | 48 | female | 人力資源 | | 3 | wupeiqi | 38 | male | 人力資源 | | 4 | yuanhao | 28 | female | 銷售 | | 5 | liwenzhou | 18 | male | 技術 | +----+-----------+------+--------+--------------+#上述sql等同于 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

3 外鏈接之左連接：優先顯示左表全部記錄

#以左表為準，即找出所有員工信息，當然包括沒有部門的員工 #本質就是：在內連接的基礎上增加左邊有右邊沒有的結果 mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id; +----+------------+--------------+ | id | name | depart_name | +----+------------+--------------+ | 1 | egon | 技術 | | 5 | liwenzhou | 技術 | | 2 | alex | 人力資源 | | 3 | wupeiqi | 人力資源 | | 4 | yuanhao | 銷售 | | 6 | jingliyang | NULL | +----+------------+--------------+

4 外鏈接之右連接：優先顯示右表全部記錄

#以右表為準，即找出所有部門信息，包括沒有員工的部門 #本質就是：在內連接的基礎上增加右邊有左邊沒有的結果 mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id; +------+-----------+--------------+ | id | name | depart_name | +------+-----------+--------------+ | 1 | egon | 技術 | | 2 | alex | 人力資源 | | 3 | wupeiqi | 人力資源 | | 4 | yuanhao | 銷售 | | 5 | liwenzhou | 技術 | | NULL | NULL | 運營 | +------+-----------+--------------+

5 全外連接：顯示左右兩個表全部記錄

全外連接：在內連接的基礎上增加左邊有右邊沒有的和右邊有左邊沒有的結果 #注意：mysql不支持全外連接 full JOIN #強調：mysql可以使用此種方式間接實現全外連接 select * from employee left join department on employee.dep_id = department.id union select * from employee right join department on employee.dep_id = department.id ; #查看結果 +------+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技術 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 6 | jingliyang | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 運營 | +------+------------+--------+------+--------+------+--------------+#注意 union與union all的區別：union會去掉相同的紀錄

三 符合條件連接查詢

#示例1：以內連接的方式查詢employee和department表，并且employee表中的age字段值必須大于25,即找出公司所有部門中年齡大于25歲的員工 select employee.name,employee.age from employee,departmentwhere employee.dep_id = department.idand age > 25;#示例2：以內連接的方式查詢employee和department表，并且以age字段的升序方式顯示 select employee.id,employee.name,employee.age,department.name from employee,departmentwhere employee.dep_id = department.idand age > 25order by age asc;

四 子查詢

#比較運算符：=、!=、>、>=、<、<=、<> #查詢平均年齡在25歲以上的部門名 select id,name from departmentwhere id in (select dep_id from employee group by dep_id having avg(age) > 25);#查看技術部員工姓名 select name from employeewhere dep_id in (select id from department where name='技術');#查看不足1人的部門名 select name from departmentwhere id in (select dep_id from employee group by dep_id having count(id) <=1);

3 帶EXISTS關鍵字的子查詢

EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。
而是返回一個真假值。True或False
當返回True時，外層查詢語句將進行查詢；當返回值為False時，外層查詢語句不進行查詢

#department表中存在dept_id=203，Ture mysql> select * from employee-> where exists-> (select id from department where id=200); +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+#department表中存在dept_id=205，False mysql> select * from employee-> where exists-> (select id from department where id=204); Empty set (0.00 sec)

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轉載于:https://www.cnblogs.com/zcfx/p/7535187.html

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